1. Field of the Invention
The present invention relates to a constant velocity joint and a wheel-support rolling bearing unit incorporating the constant velocity joint.
A wheel-support rolling bearing unit according to the present invention is a so-called fourth-generation hub unit, and utilized for supporting the drive wheels {(which imply front wheels of an FF car (front-engine front-drive car), rear wheels of an FR car (front-engine rear-drive car) of an RR car (rear-engine rear drive car), and whole wheels of 4WD car (four-wheel drive car)} held on the independent suspension so that the drive wheels are rotatable about the suspension.
A constant velocity joint according to the present invention is integrally incorporated into a rolling bearing unit for supporting drive wheels on, e.g., an independent suspension, and utilized for transmitting a driving force from a transmission to the drive wheels.
2. Related Background Art
A constant velocity joint is provided between a transmission of an automobile and a drive wheel supported on an independent suspension, whereby a driving force (traction) of an engine is transmittable to the drive wheel at the same angular speed along an entire periphery irrespective of a relative displacement between a differential gear and the drive wheel and of a steering angle given to the wheel. What has hitherto been known as the constant velocity joint used for such a mechanism, is disclosed, e.g., U.S. Pat. No. 3,324,682, U.S. Pat. No. 3,412,580 and U.S. Pat. No. 4,589,857.
This type of constant velocity joint 1 which has been known so far is constructed so that a rotary force is, as shown in, e.g., FIGS. 21-23, transmitted between an inner race 2 and an outer race 3 through six pieces of balls 4, 4. The inner race 2 is fixed to an external side end (a left side end in FIG. 21) of one shaft 5 rotationally driven by the transmission. Further the outer race 3 is fixed to an internal side end (a right side end in FIG. 21) of another shaft 6 for fixing the drive wheel. Six streaks of inner engagement grooves 7, 7 each taking a circular arc shape in section are formed in an outer peripheral surface 2a of the inner race 2 in a direction right-angled to a circumferential direction at an equal interval in the circumferential direction. Six streaks of outer engagement grooves 8, 8 each taking the circular arc shape in section are likewise formed in an outer peripheral surface 3a of the outer race 3 in positions facing to the inner engagement grooves 7, 7 in the direction right-angled to the circumferential direction.
A cage 9 assuming a circular arc shape in section but an annular shape on the whole is sandwiched in between the outer peripheral surface 2a of the inner race 2 and the inner peripheral surface 3a of the outer race 3. Pockets 10, 10 are formed in positions aligned with the two groups of inner and outer engagement grooves 7, 8 as well as in six positions in the circumferential direction of the cage 9, and totally six pieces of balls 4, 4 are held one by one inwardly of each of the pockets 10, 10. These balls 4, 4 are capable of rolling along the two groups of inner and outer engagement grooves 7, 8 in a state of being held in the pockets 10, 10.
The pockets 10, 10 are, as illustrated in FIG. 23, each takes a rectangular shape elongated in the circumferential direction, and structured to, even when a spacing between the balls 4, 4 adjacent to each other in the circumferential direction might change with a variation in an axial crossing angle .alpha. which will hereinafter be explained, absorb this change. Namely, a positional relationship between bottom surfaces 7a, 7a of the inner engagement grooves 7, 7 and a positional relationship between bottom surfaces 8a, 8a of the outer engagement grooves 8, 8, become such as a relationship of the longitude lines on a globe as indicated by the one-dotted chain line in FIG. 24. If the central axis of the inner race 2 is concentric with the central axis of the outer race (the axial crossing angle .alpha.=180.degree.), each of the balls 4, 4 exists in the vicinity of a position corresponding to the equator on the globe which is indicated by the two-dotted line in FIG. 24. Whereas if the central axis of the inner race 2 is not concentric with the central axis of the outer race (the axial crossing angle .alpha.&lt;180.degree.), the balls 4, 4 displace in reciprocation (displace alternately in the direction of the North Pole and in the direction of the South Pole on the globe) in the up-and-down direction in FIG. 24 with a rotation of the constant velocity joint 1. As a result, the spacing between the balls 4, 4 adjacent to each other in the circumferential direction changes, and hence the pockets 10, 10 each takes the rectangular shape elongated in the circumferential direction, thereby enabling the spacing therebetween to change. Note that the bottom surfaces 7a, 7a of the inner engagement grooves 7, 7 and the bottom surfaces 8a, 8a of the outer engagement grooves 8, 8, are not concentric with each other as obvious from the explanation which follows. Accordingly, the lines corresponding to the longitude lines exist in positions slightly deviating from each other for every corresponding engagement groove 7 or 8.
Further, as shown in FIG. 21, the balls 4, 4 are disposed within a bisection plane c which bisects the axial crossing angle .alpha. between the two shafts 5, 6, i.e., the angle .alpha. made by two lines a and b at a point-of-intersection O between a central line a of one shaft 5 and a central line b of the other shaft 6. Therefore, the bottom surfaces 7a, 7a of the inner engagement grooves 7, 7 are located on a spherical surface wherein a point d existing away by h from the point-of-intersection O on the central line a is centered, and the bottom surfaces 8a, 8a of the inner engagement grooves 8, 8 are located on a spherical surface wherein a point e existing away by h from the point-of-intersection o on the central line b is centered. The outer peripheral surface 2a of the inner race 2, the inner peripheral surface 3a of the outer race and two inner and outer peripheral surfaces of the cage 9, are, however, located on the spherical surface with the point-of-intersection O being centered, thereby enabling the outer peripheral surface 2a of the inner race 2 and the inner peripheral surface of the cage 9 to slide on each other, and also the outer peripheral surface 3a of the outer race 3 and the outer peripheral surface of the cage 9 to slide on each other.
In the case of the thus constructed constant velocity joint 1, when the inner race 2 is rotated by one shaft 5, this rotary motion is transmitted via the six balls 4, 4 to the outer race 3, whereby the other shaft 6 rotates. If a positional relationship (which implies the axial crossing angle .alpha.) between the two shafts 5, 6 changes, the balls 4, 4 roll along the two groups of inner and outer engagement grooves, thus allowing the displacement between one shaft 5 and the other shaft 6.
The basic structure and operation of the constant velocity joint are as described above. The basic structure and operation of the constant velocity joint which have been explained referring to FIG. 21 are applied to the present invention and the embodiments thereof which will be discussed later on.
On the other hand, it has been a technical pursuit over the recent years that the constant velocity joint described above is combined integrally with a wheel-support rolling bearing unit for rotatably supporting the wheel on a suspension. Namely, the operation of rotatably supporting the wheel of an automobile on the suspension involves the use of the wheel-support rolling bearing unit in which the outer race and the inner race are rotatably combined through rolling members. If the thus constructed wheel-support rolling bearing unit is combined integrally with the above-described constant velocity joint, the wheel-support rolling bearing unit and the constant velocity joint can be so constructed as to be downsized and to reduce weights thereof on the whole. What has hitherto been well known as the wheel-support rolling bearing unit, i.e., a so-called fourth-generation hub unit structured to integrally combine the wheel-support rolling bearing unit with the constant velocity joint, is disclosed in Japanese Patent Application Laid-Open Publication No. 7-317754.
FIG. 25 shows a prior art structure disclosed in the same Publication. An outer race 11, which does not rotate in a state of being supported on the suspension as well as in a state of being assembled to a vehicle, includes a first fitting flange 12, formed on an outer peripheral surface thereof, for supporting the wheel on the suspension, and plural trains of outer race tracks 13, 13 formed along an inner peripheral surface, respectively. A hub 16 constructed by combining first and second inner race members 14, 15 is disposed inwardly of the outer race-11. The first inner race member 14 of these first and second inner race members 14, 15 is formed in a cylindrical configuration and includes a second fitting flange 17, provided at a portion, closer to one side end (on a left side in FIG. 25), on the outer peripheral surface, for supporting the wheel, and a first inner race track 18 provided at a portion closer to the other side end (on a right side in FIG. 25), respectively. While on the other hand, the second inner race member 15 includes a cylindrical portion 19, provided at one side end (a left side end in FIG. 25), for externally fixedly fitting the first inner race member 14, an outer race 3A for a constant velocity joint 1a, which is provided at the other side end (a right side end in FIG. 25), and a second inner race track 20 formed in an outer peripheral surface of an intermediate portion. Then, a plurality of rolling members 21 and another plurality of rolling members 21 are provided between the outer race tracks 13, 13 and the first and second inner race tracks 18, 20, whereby the hub 16 is rotatably supported inwardly of the outer race 11.
Further, engagement grooves 22, 23 are formed in positions aligned with each other on the inner peripheral surface of the first inner race member 14 and on the outer peripheral surface of the second inner race member 15, and a stop ring 24 is provided in a state of bridging the two engagement grooves 22, 23, thus preventing the first inner race member 14 from coming off the second inner race member 15. Further, a portion between an outer peripheral edge of one side end surface (a left side end surface in FIG. 25) of the second inner race member 15 and an inner peripheral edge of a stepped portion 25 formed on the inner peripheral surface of the first inner race member 14, is welded 26, thereby fixedly joining the first and second inner race members 14, 15 to each other.
Moreover, covers 27a, 27b each taking substantially a cylindrical shape and composed of a metal such as a stainless steel etc and annular seal rings 28a, 28b each composed of an elastic material such as elastomer like a rubber, are provided between openings formed at both side ends of the outer race 11 and the outer peripheral surface of the intermediate portion of the hub 16. The covers 27a, 27b and the seal rings 28a, 28b cut off the portions provided with the plurality of rolling members 21, 21 from outside, thereby preventing grease existing in those portions from leaking outside and also preventing foreign matters such as rain water and dusts etc from permeating those portions. Moreover, a screen board 29 for closing the inside of the second inner race member 15 is provided inwardly of the intermediate portion of the second inner race member 15, thereby ensuring a rigidity of the second inner race member 15 and preventing the foreign matters from arriving at the constant velocity joint 1a, which have entered the interior of the second inner race member 15 from an opening at the front side end (a left side end in FIG. 25) of the second inner race member 15. Note that the constant velocity joint 1a is constructed in the same way as that of the constant velocity joint 1 previously illustrated in FIGS. 21-23.
When assembling the thus constructed wheel-support rolling bearing unit to the vehicle, the outer race 11 is supported through the first fitting flange 12 on the suspension, and the wheel defined as a drive wheel is fixed through the second fitting flange 17 to the first inner race member 14. Further, a front side end of an unillustrated drive shaft rotationally driven by an engine through a transmission, is spline-engaged with the inside of the inner race 2 constituting the constant velocity joint 1a. When the automobile travels, rotations of this inner race 2 are transmitted via the plurality of balls 4 to the hub 16 including the second inner race member 15, thereby rotationally driving the wheel.
For attaining further downsizing of the fourth-generation wheel-support rolling bearing unit described above, it is effective to reduce a diameter of a circumscribing circle of each of the plurality of balls 4, 4 constituting the constant velocity joint 1a. Then, the diameter of each of the balls 4, 4 is reduced for decreasing the diameter of the circumscribing circle, and besides it is required for securing a torque transmittable through the constant velocity joint 1a that the number of the balls 4, 4 be increased. Moreover, under such circumstances, even when increasing the number of the balls 4, 4, there might be a necessity for ensuring strength and durability of each of column members 30, 30 (see FIGS. 22, 23, 27 and 29 to 31) existing between the plurality of pockets 10, 10 provided in the cage 9 in order to secure a durability of the cage 9 for holding the respective balls 4, 4.
The reason why when the number of the balls 4, 4 is increased from 6 up to 8, there rises a rate of the balls occupying the cage in the circumferential direction even if a major diameter D.sub.a is reduced to some extent. As a result, a circumference-directional width of each of the column members 30, 30 (FIGS. 22 and 23) existing between the pockets 10, 10 adjacent to each other in the circumferential direction, is narrowed, and there is a deficiency in terms of a rigidity of the cage 9, which might lead to a possibility in which damages such as cracks etc occur at a peripheral edge of each of the pockets 10, 10 with a long-term use. Namely, if the constant velocity joint 1a is operated in a state of giving a joint angle (at which a positional relationship between the central axis of the inner race 2 and the central axis of the outer race 3A deviates from a rectilinearity, i.e., a supplementary angle of the axial crossing angle .alpha. shown in FIG. 21), the respective balls 4, 4 receive forces as indicated by arrowheads a, a in FIGS. 26 and 27 from the bottom surfaces 7a, 8a of the two inner and outer engagement grooves 7, 8. Then, the balls 4, 4 are pressed by a resultant force of the forces indicated by the arrowheads a, a against an intermediate portion of an inner surface of the rim portion 31 of the cage 9. As a result, a moment load, with a connecting portion to the column members 30, 30 being centered, is applied to the rim portion 31, and a stress is applied to this connecting portion. This stress becomes greater as a length of each of the pockets 10, 10 in the circumferential direction becomes larger, and as the length dimension of each of the column members 30, 30 in the circumferential direction becomes smaller, with the result that the connecting portion is easily damaged like cracks etc. Such being the case, it is required for ensuring the ample durability of the cage 9 that the length dimension of each of the pockets 10, 10 in the circumferential direction be reduced, and that the length dimension, in the circumferential direction, of each of the column members 30, 30 adjacent to each other in the circumferential direction be increased.
The process of increasing the length dimension of each of those column members 30, 30 is controlled in terms of preventing interference with the balls 4, 4. To be more specific, first, the length of each of the pockets 10, 10 in the circumferential direction needs, when rotating the constant velocity joint 1a in the state of giving the joint angle, to be large enough to enable each of the balls 4, 4 to displace in the circumferential direction of the cage 9. Second, the above length must be, after assembling together the inner race 2, the outer race 3A and the cage 9 in order to assemble the constant velocity joint 1a, large enough to incorporate the balls 4, 4 into the pockets 10, 10 of the cage 9.
European Patent 0 802 341 A1 discloses the constant velocity joint 1b as shown in FIGS. 28-31 by way of a structure for increasing the length dimension of each of the column members 30, 30 while setting the number of the balls 4, 4 to 6 or larger in consideration of the above point. The constant velocity joint 1b disclosed in the above Publication is structured to transmit the rotary force between the inner race 2 and the outer race 3 through eight pieces of balls 4, 4. Then, in the case of the structure disclosed in the same Publication, two types of pockets 10a, 10b each having a different length dimension in the circumferential direction, are disposed alternately at an equal interval in the circumferential direction. With this arrangement, as compared with the case of using the single type of pockets, it is feasible to increase a circumference-directional width of each of the column members 30, 30 existing between the pockets adjacent to each other in the circumferential direction. There is made, however, no contrivance about the width of each of the column members 30, 30 in terms of securing the durability of the cage 9a while ensuring life-spans of other components of the constant velocity joint 1b.
In other words, there is made no contrivance of optimally controlling a relationship between the major diameter of each of the balls 4, 4 constituting the constant velocity joint 1b and the width of each of the column members 30, 30, considering a relationship between the rolling fatigue line-span of each of the inner and outer engagement grooves 7, 8 and the strength of the cage 9a. The above Publication does not disclose such a point at all that the constant velocity joint 1b is designed in consideration of the above point.
In the case of the above-described structure disclosed in the European Patent 0 802 341 A1, each of the balls 4, 4 is held in each of the pockets 10a, 10b, and hence it is difficult to equilibrate at a high level the major diameter and the number of the balls 4, 4 and the length dimension of each of the column members 30, 30 when ensuring these factors. Therefore, the constant velocity joint capable of transmitting sufficiently a large torque and exhibiting an enough durability can not be necessarily actualized.
It can be considered to enlarge a section area of each of the column members 30, 30 by increasing a thickness of the cage 9 for securing the strength and the durability thereof even when the width of each of the column members 30, 30 is small.
There arises, however, a fresh problem which follows, if the major diameter of the cage is increased or if a minor diameter thereof is decreased in order to enlarge the sectional area.
First, the increase in the major diameter of the cage leads to a rise in a diameter of an inner peripheral surface 3a of the outer race 3 (3A). This rise in the diameter of the inner peripheral surface 3a leads to a decrease in depth of the outer engagement groove 8. Similarly, a decrease in the minor diameter of the cage leads to a reduction in a diameter of an outer peripheral surface 2a of the inner race 2. This decrease in the diameter of the outer peripheral surface 2a leads to a decrease in depth of the inner engagement groove 7.
When the depth of each of the two groups of outer and inner engagement grooves 8, 7 decreases, there is lessened the rigidity of the constant velocity joint 1 (11a) in a rotational direction, which is based on an engagement of each of the balls 4, 4 with each of the two groups of engagement grooves 8, 7. Further, when transmitting a large torque between the inner race 2 and the outer race 3 (3A), a rolling surface of each ball 4 becomes easier to run on an opening edge of each of the engagement grooves 8, 7. As a result, the durability of the constant velocity joint is ensured with the difficulty because of a shortened rolling fatigue life-span of the rolling surface of each ball 4, and so forth.
Accordingly, it must be controlled in terms of obtaining a required depth of the engagement groove that the major diameter of the cage 9 is increased or that the minor diameter thereof is reduced.
On the other hand, it is also required that a minimum thickness of the cage be controlled in terms of ensuring the durability of the constant velocity joint 1 (1a). Namely, if the cage 9 is composed of a material having a large strength such as, e.g., a high-function resin and a high-tension steel etc., the strength and the durability of the column member 30 itself can be ensured. In this case also, however, if the thickness thereof is too small, the following problem might arise.
That is, as obvious from the discussion given above, during an operation of the rzeppa type constant velocity joint 1 (1a) at which the present invention aims, the balls 4, 4 displace in the diametrical direction of the cage 9 as well as in the circumferential direction thereof. With such a displacement, when a maximum-major-diameter portion of the ball 4 impinges upon the opening edge of the pocket holding the ball inside, there might be a possibility wherein this opening edge is chipped off.
To begin with, if the major diameter of the cage 9 is too small, the maximum-major-diameter portion of the ball 4 existing upward in FIG. 21 impinges upon the peripheral edge of the opening on the side of the major diameter of the pocket 10. Whereas if the minor diameter of the cage 9 is too large, the maximum-major-diameter portion of the ball 4 existing downward in FIG. 21 impinges upon the peripheral edge of the opening on the side of the minor diameter of the pocket 10. Every opening peripheral edge of the pocket 10 takes a sharp configuration and might be therefore, if strongly pressed by the rolling surface of the ball 4, chipped into minute fragments. A sectional configuration of the opening peripheral edge on the side of the major diameter has an acute angle especially when the pocket 10 is formed by punch-out working, and hence, if the cage 9 is composed of a steel subjected to a hardening process, the above chips might be easily produced.
Then, if the chips enter between the balls 4, 4 and the engagement grooves 8, 9, the inner surfaces of the engagement grooves 8, 7 and the rolling surfaces of the balls 4, 4 are damaged, which in turn causes a decline of the durability of the constant velocity joint 1 (1a)
Accordingly, it must be controlled in terms of preventing the rolling surfaces of the balls 4, 4 from impinging upon the opening peripheral edges of the pockets 10 to reduce the major diameter of the cage 9 or to increase the minor diameter thereof.
As described above, it is required that the maximum and minim of the major and minor diameters of the cage be controlled in terms of securing the rigidity and the durability of the constant velocity joint 1 (1a), however, no contrivance on this point has been made in the prior art.
Further, the specification of British Patent No. 1,537,067 discloses a structure in which the balls 4, 4 are, as shown in FIG. 32, held by twos in each of three pockets 10c, 10c formed in positions at an equal interval in the circumferential direction of a cage 9b. According to this structure, a length dimension of each of column members 30, 30 existing between the pockets 10c, 10c adjacent to each other in the circumferential direction, is increased corresponding to a degree to which the interval between the balls 4, 4 held in the same pocket 10c, thereby ensuring a durability of the cage 9b.
In the case of the above-mentioned structure disclosed in the specification of British Patent 1,537,067, no consideration is made with respect to the strength of the cage.
Further, as explained above, it is necessary for attaining the downsizing and the reduction in weight of the wheel-support rolling bearing unit known as the so-called fourth-generation hub unit to reduce, as shown in FIG. 25, a major diameter of a housing unit 3A by decreasing the major diameter of each of the balls 4, 4 constituting the constant velocity joint 1a and thus decreasing the diameter of the circumscribing circle of the balls 4, 4. Then, there is a necessity for ensuring a load capacity of the constant velocity joint 1a by reducing the major diameter of each of the balls 4, 4 and increasing the number of the balls 4, 4 (from 6 to 7 or more).
If the major diameter of each of the balls 4, 4 is set too small, however, there might decrease a contact ellipse existing at impingement portions between the rolling surfaces of these balls 4, 4 and the inner surfaces of the inner engagement groove 7a and the outer engagement groove 8a, with the result that a surface pressure upon those impingement portions becomes excessively large. The rolling fatigue life-span of the inner surface of each of the engagement grooves 7a, 8a is thereby shortened. If the major diameter of each of the balls 4, 4 is simply increased for preventing the reduction in the rolling fatigue life-span of the inner surface of each of the engagement grooves 7a, 8a due to the above cause, the interval between the balls 4, 4 adjacent to each other in the circumferential direction is narrowed. Then, there is decreased the width of each of the column members existing between the pockets 10, 10 for holding the balls 4, 4 with respect to the cage 9. The reduction in the width of the column member is not also preferable because of leading to the decline of the durability of the cage 9.
If the major diameter of the housing unit 3A is increased, the major diameter of each of the balls 4, 4 is also increased, and besides the width of each column member can be ensured. It is, however, impossible to attain the downsizing and the reduction in the weight of the wheel-support rolling bearing unit called the fourth-generation hub unit, which is not preferable.